I. Field of the Invention
This invention relates generally to printing and, more particularly, to thermal transfer printing suited to impart an indicia to a workpiece, e.g., a mail envelope.
Thermal transfer printing of an image to a workpiece is a known technology. Generally, thermal transfer printing utilizes a thermal print head consisting of a linear array of "ON-OFF" heating elements. Each element can be individually actuated in binary response to a generated bit input signal. Customarily, a control signal is generated by a control means, such as a programmable microcomputer, wherein a series of byte codes are transmitted to the thermal print head gating the individual heating elements to either an "ON" or "OFF" state in response to the control signal. A thermal ribbon coated on one side with thermally sensitive ink is passed between the thermal print head and a traversing workpiece. In response to the gating pattern of the print head elements, a series of dots and spaces are created on the workpiece. As the gate information is sequentially transmitted to the thermal head in synchronized relationship to the traversing thermal ribbon and workpiece, an image is thereby imprinted to the workpiece.
Thermal transfer printing offers a most important advantage over die cast image transfer techniques, in that images transferred by thermal transfer printing have a superior resolution quality. However, thermal transfer printing quality is sensitive to the workpiece image transfer surface area texture or roughness. The thermal transfer quality is limited by the capability of the thermal head to be subjected to high compression loads. High compression loads lead to shortened thermal head life. Therefore, the contact pressure between the workpiece, thermal ribbon, and thermal head must be maintained at a relatively low level. On the other hand, a workpiece having a rough surface texture has reduced surface contact with the thermal ribbon due to variations in evenness of surface contact by the ribbon, as compared with evenness of contact with a surface having a smooth surface area. As a result, a workpiece with a rough surface texture must be subjected to higher thermal head pressure to avoid providing an image lacking in resolution and contrast.
It would be advantageous if rough workpieces could be imaged by thermal transfer techniques in a manner preserving the superior imaging capabilities of thermal printing. In addition thermal transfer printers are programmable. The programmable capability of thermal transfer printing systems allows imaging flexibility which is not achievable with conventional die cast methods.
Substantial efforts have heretofore been made to remove or despoil the image remaining on the ink donor ribbon after printing has been performed on the workpiece. These attempts have been made to deny reuse of the ribbon. Examples of such efforts are provided by U.S. Pat. No. 4,407,002 issued Sep. 27, 1983 to Inui et al and U.S. Pat. No. 4,511,902 issued Apr. 16, 1985 to Nagashima. Drawbacks of processes and apparatus for attaining this goal is that speed is reduced, and that the apparatus and the process is necessarily more complex and, therefore, more expensive.
It is with knowledge of the prior art and the shortcomings thereof that the present invention has been conceived and is now reduced to practice.